This supplemental application requests funds for improving synchrony of animal cells in culture, and for determining exactly the timing of certain G1 events. I have discovered that certain drugs reversibly stop the growth of normal BHK cells in tissue culture, and these drugs do not similarly stop transformed, malignant cells derived from the normal ones. This difference has potential applications for cancer chemotherapy which are being explored. A better understanding of the events that occur when the normal cells are arrested will allow us to select drugs that differentiate between normal and cancer cells. The drugs arrest normal cells somewhere in the part of the cell cycle between division and start of DNA synthesis, namely G. I want to discover just when in G1 the drugs are active, and how this time is related to the times of cell division, start of DNA synthesis, and the time at which resting cells start to grow again. Also, the individual cells in a population traverse G1 in very different times. I want to dimish variablity as much as possible, and learn where this variability is located in G1. A precise knowledge of timing will helps us in looking at the proper moment for biochemical changes associated with action of the drugs. We will examine various media to obtain optimal synchrony. Thus, the experiments will consist of synchronizing populations of cells in culture and determining by autoradiography at what times in the cell cycle the drugs are most effective in blocking DNA initiation. Combinations of the data will allow us to make a map of the main events in the G1 part of the cycle, in relation to the drugs' action. Preliminary experiments show that the approach is feasible.